Method for welding conductors

ABSTRACT

The invention relates to a method for ultrasonically welding electrical conductors. According to said method, the conductors ( 32 ) are introduced into a compression space ( 30 ) that is delimited by at least two confining elements and are welded together after closing the compression space, ultrasound being applied via a sonotrode ( 16 ) and the conductors which are to be welded being impinged upon by pressure via a counterelectrode ( 18 ). In order to be able to verify the quality of the welding point by using simple measures, the pressure in the compression space ( 30 ) is relieved following welding ( 32 ) of the conductors, whereupon an ultrasonic pulse is applied to the welded conductors while the same are impinged upon by pressure, and a modification in the distance between the sonotrode ( 16 ) and the counterelectrode ( 18 ) is measured thereafter.

The invention concerns a method for welding electric conductors such aslitz wires using ultrasound, especially litz wires among one another toproduce transit or end nodes, or litz wires with a support, wherein theconductors are introduced into a compression chamber bounded by at leasttwo bounding elements and are welded after the compression chamber isclosed, wherein ultrasound is applied via a first element such as asonotrode and the conductors to be welded are subjected to pressure viathe first element or a second element such as a counter electrode, andwherein a characteristic magnitude of the compression chamber ismeasured. The invention also relates to a method for the qualityverification of conductors welded in a compression chamber of anultrasound welding device, especially litz wires welded into a transitor an end node, or litz wires welded on a carrier, whereby theconductors to be welded are arranged at least between a first elementapplying ultrasound, such as a sonotrode, and a second element, such asa counter electrode, whereby the conductors are subjected to pressurevia the second or the first element. Finally the invention relates to amethod for verifying the quality of welded conductors, especially weldedlitz wires such as transit or end nodes or litz wires welded on asupport.

In the ultrasound welding of a node, several litz wires, such as copperconduits, which in turn consist of several strands, are introduced intoa compression chamber, compressed, and then brought into relativevibration toward one another by ultrasound. The friction of the strandsamong one another leads to a welding of the surfaces so that a solidnode exists after the welding. During the compression and welding, thevolume of the compression chamber diminishes. This change in volume canbe measured by displacement pickups and used as a comparison value forchecking quality. The strength of the node is the decisive qualitycriterion for the process of ultrasound welding.

In order to determine the strength of corresponding nodes, the nodes canbe measured destructively or non-destructively, by pulling or peeling,via a type of hardness testing (splice checker).

One method for regulating the course of the process for quality controlin the ultrasound welding of workpieces can be found in EP-B-0 208 310.In this method, in activating the ultrasound energy, a sonotrode of anultrasound welding device is lowered onto a workpiece lying on an anvil,whereby a null balance takes place when the sonotrode is placed upon theworkpiece, in order to subsequently activate the sonotrode and to allowa desired deformation path to pass through.

A method for the ultrasound welding of thermoplastic materials is knownfrom SU-A-757 337, in which the depression depth of the sonotrode, andtherewith the deformation of the workpieces during the welding process,is recorded via a displacement pickup directly applied to the sonotrode.The welding process is terminated with a sign change of the secondtemporal derivation of the path signal.

A method, a device, and a system for producing connections to integratedcircuits are described in DE-A-2 149 748. In order to attain improvedconnections, the introduction of force onto a tool, but also the energyacting upon it, is controlled via the deformation of the parts to beconnected, via a displacement pickup.

A regulation of the welding process using ultrasound is known fromFR-A-2 302 172 in which the speed of the depression motion of asonotrode during the welding process is recorded, compared withspecified limit values, and the energy supply to the sonotrode isrestricted when adjustable thresholds are exceeded.

In WO-A-95/10866 A method and a device for welding conductors aredescribed, with which a defined welding can take place independently ofcross section, even when conductors of different cross sections aresuccessively welded in random order. For this, a characteristicmagnitude of the compression chamber is measured after compression ofthe conductors to be welded.

In order to be able to adjust the height and width of a compressionchamber that accommodates conductors to be welded, and to be able to setthe respective cross section automatically, devices are known, such asthose disclosed in EP-B-0 143 936 or DE-C-37 19 083.

The present invention is based upon the problem of further developing amethod of the type mentioned at the beginning such that immediatelyafter the node is produced, it can be checked whether the weldingsatisfies desired quality standards. The invention also relates to amethod for verifying the quality of welded conductors.

The problem is essentially solved in accordance with the invention inthat the compression chamber is decompressed after the conductors arewelded, and ultrasound is supplied on the welded conductors, after whichthe characteristic magnitude is measured.

The quality of the welding is checked in accordance with the inventiondirectly in the compression chamber without additional apparatus beingrequired. For this, it is provided in accordance with the invention thatthe compression chamber is decompressed after welding in order then toprovide a short sound impulse on the weld as well as the nodes. If theweld is not solid, the conductors or litz wires that have been placed invibration by the sound will travel toward the boundary of thedecompressed compression chamber. The boundary can consequently yield sothat a relatively large change in the path of the second element takesplace on the basis of the pressure applied continuously via the secondelement, such as a counter electrode or anvil. This measure of thechange in path, which can be recorded via a displacement pickup,consequently provides information on whether or not the welding meetsquality standards. For if only a slight change in path takes place, itcan be concluded that the weld has the requisite strength.

Decompression generally means that, for example, a boundary or lateralelement of the compression chamber is unlatched or released so that thelateral element is displaced on the basis of the node becoming “soft”yielding to the pressure of the second element, to the extent that thenode does not have the requisite strength. As a result of the yieldingof the bounding element, the soft node almost flows so that the secondelement is correspondingly shifted in the direction of the firstelement. Consequently a characteristic change in path of the secondelement takes place when the node is not sufficiently strong. Theyielding of the bounding element, thus its change in path, can also beused as a characteristic magnitude for drawing inferences about thequality of the node.

By selecting suitable parameters during the test, such as, for example,pressing force, amplitude or duration of the sound, it is possible todifferentiate on the basis of the amount of the change in path whetheror not the weld or the node is firm, and thus whether or not itsatisfies the desired quality standards.

The decompression of the compression chamber in particular takes placein that the compression chamber is bounded by at least three elements,and after welding at least one element of the welded conductors isunlocked or released to the requisite extent. Even moving the element toopen the compression chamber fulfills the feature of decompression.

In particular a geometric value such as height, width or diagonals ofthe compression chamber is to be selected as the characteristicmagnitude that enables inferences as to the quality of the weld, wherebythe geometric value is recorded by a displacement pickup, for example.

In measuring the diagonals, the compression chamber is moreoverapparently closed on its peripheral side. Nonetheless, especially thespacing between the ultrasound-emitting first element (sonotrode) andthe pressure-transmitting second element (anvil or counter electrode) isselected as a characteristic magnitude.

In other words, the invention is basically related to a method forwelding electric conductors using ultrasound, whereby the conductors areintroduced into a compression chamber bounded by at least two boundingelements, and are welded after closing the compression chamber, wherebyultrasound or ultrasound vibrations are applied via a sonotrode and theconductors to be welded are subjected to pressure via a counterelectrode. In order to be able to check the quality of the welding pointusing simple methods, it is proposed that the compression chamber bedecompressed following the welding of the conductors, and that then anultrasound pulse or ultrasound vibration pulse be applied to the weldedconductors with simultaneous action of pressure to these, and thatsubsequently the change in spacing of the sonotrode and counterelectrode be measured. While the conductors are being subjected topressure, one of the bounding elements can yield to the weldedconductors due to decompression of the compression chamber, such asreleasing one of the bounding elements, to the extent that theconductors experience a deformation in the direction of the boundingelement.

A method for checking the quality of welded conductors of the typementioned at the beginning is in particular distinguished by thefollowing steps:

-   -   Introducing the welded conductors into a compression chamber        bounded by at least the first and second elements,    -   Compacting and welding the conductors with simultaneous cross        section diminution of the compression chamber,    -   Decompressing the compression chamber, wherein the welded        conductors remain between the first element and the second        element,    -   Renewed application of ultrasound with simultaneous action of        pressure on the welded conductors and    -   Measurement of a characteristic magnitude of the compression        chamber and/or the shape of the welded conductors.

Here the quality of the welding is evaluated as a function of themeasured characteristic magnitude of the compression chamber and/or theshape of the welded conductors.

In particular, height and/or width and/or diagonals of the compressionchamber can be measured as characteristic magnitudes of the compressionchamber using a displacement pickup, for example.

The renewed application of the ultrasound should take place over aduration T with 10 ms≦T≦250 ms. Moreover, the renewed subjection toultrasound should follow immediately on the welding process, almostcontinuing the latter.

In order to ensure that lower quality welds cannot flow into a normalfinishing process, a further development of the invention, which is tobe emphasized, provides that when a lower quality weld is established onthe welded conductors while the open compression chamber continues to bedecompressed, ultrasound is applied again to destroy or largely destroythe weld.

If a proper weld is established, a targeted recompression can take placewhile the compression chamber is open to increase the strength of theweld.

The method of quality control of the invention can nonetheless beconducted independently of the device in which the conductors arewelded, even if preferably the welding and the checking of the weldedconductors, such as especially litz wires welded to transit and endnodes, takes place in one and the same device.

Thus the welded conductors can be arranged between a first elementapplying ultrasound vibrations, such as a sonotrode, and a secondelement, such as a counter electrode. Then ultrasound or ultrasoundvibrations are applied and the change in spacing between the first andthe second element with simultaneous application of pressure to thewelded conductors that took place during or after the application can bemeasured. An evaluation as to whether the welded conductors correspondto the quality standards is then performed in the previously describedmanner on the basis of the change in spacing between the first and thesecond element. Should a change in spacing between the first and secondelement take place in an impermissible manner, the conclusion may bedrawn that the welded connection does not meet the standards imposed.

With regard to the quality verification described above, parameters andmeasuring methods that have been discussed above are applied forchecking the quality of the welded conductors.

Further particularities, advantages and features of the invention emergenot only from the claims, the features to be inferred from the latter-bythemselves and/or in combination-but also on the basis of the preferredembodiments to be inferred from the subsequent description of thedrawings, wherein:

FIG. 1 Is a basic representation of an ultrasound welding arrangement,

FIG. 2 Represents a compression chamber of an ultrasound welding devicein an initial position,

FIG. 3 Illustrates the compression chamber in accordance with FIG. 2 ina second position and

FIG. 4 Shows a further embodiment of the compression chamber.

An arrangement is represented purely in outline in FIG. 1 with which inparticular electrical conductors are welded using ultrasound into end ortransit nodes. The arrangement includes an ultrasound welding device ormachine 10 which in the usual manner includes a converter 12, or if needbe a booster 14 as well as a sonotrode 16. The sonotrode 16 or a surfaceof this is allocated a multipart counter electrode 18 (also called ananvil) as well as a slider 20, as can be taken from DE-C-37 19 083, tothe disclosure of which reference is expressly made. The sonotrode 16 orits surface, the counter electrode 18, and the slider 20 bound acompression chamber adjustable in cross section, which is explained ingreater detail on the basis of FIG. 2-4. The elements to be welded, suchas conductors, are introduced into the compression chamber.

The converter 12 is connected via a conductor 22 with a generator 24,which in turn leads via a line 26 to a PC 28, which is used to controlthe welding process and in which welding parameters or cross section inrelation to conductors to be welded can be input or corresponding storedvalues can be called up.

As is apparent from FIG. 2, the ultrasound welding device 10 has acompression chamber 30 that is bounded by the sonotrode 16, the counterelectrode 18, and the lateral slider 20, in which conductors 32 to bewelded are introduced in the embodiment. The counter electrode 18consists of a vertically adjustable column or plate 34 from which ahorizontally displaceable transverse slider 36 proceeds. Furthermore, adisplacement pickup 38 is allocated to the vertically adjustable plate34. The motion of the vertical plate 34, the transverse slider 36, andthe lateral slider 20 is symbolized by the double arrows 40, 42, 44.

Once the conductors 32 have been introduced into the compression chamber30, the lateral slider 20 is first moved in the direction of theconductor 32. The transverse slider 36 is correspondingly positioned sothat this can be moved along the slider 20 when the plate 34 is movedvertically, that is, along its surface 46 that borders the compressionchamber 30. The lower boundary surface of the compression chamber 30 isformed by a surface 48 of the sonotrode 16. The opposite boundarysurface is a surface 50 of the transverse slider 36. The remainingboundary surface 52 of the compression chamber 30, which runs parallelto the boundary surface 46, is formed by the vertical plate 34.

When the compression chamber 30 is closed, first a compacting of theconductors 32 and then a welding of these takes place in which thesonotrode 16 is placed in ultrasound vibration. At the same time thecounter electrode 18-also called the anvil-is adjusted in the directionof the arrow 40 in the direction of the boundary surface 48 of thesonotrode 16 (arrow 40), whereby a requisite force or pressure actionupon the conductors 32 takes place.

Once the welding process has been completed, thus once a node 54 isproduced, the lateral slider 20 is relieved of pressure according to therepresentation of FIG. 3. This can also take place through the unlockingor release of the lateral slider 20 so that the latter can be shifted inthe direction of the arrow 49 via forces transmitted to the welded nodes54. A decompression can also take place in that the compression chamber30 is opened by adjusting the lateral slider 20. Then ultrasound or anultrasound pulse with suitable amplitude and duration is transmitted viathe sonotrode 16 to the nodes 54 with the further application ofpressure to the nodes 54 via the counter electrode 18 or the transverseslider 36. In this way, the conductors or the litz wires of the nodes 54can yield in the direction of the lateral slider 20, to an extent thatis dependent upon the degree to which the node 54 possesses therequisite strength. Since there exists an action of force on the nodes54 via the transverse slider 36, when the conductors or litz wires yieldto this motion, a vertical motion of the counter electrode 18 takesplace, which is measured by the displacement pickup 38 and forwarded toa control unit 56. On the basis of the change in path ascertained viathe displacement pickup 38, it can be established via the control unit56 whether or not the node 54 possesses the required strength andthereby the necessary quality. For with a sufficient strength of thenode 54, only a minimal motion of the counter electrode 18 is measuredby the displacement pickup 38.

Thus there results, for example, with a node having a cross section of1.5 mm² a path change of ca 0.05 mm if the requisite strength exists. Ifthis is not the case there results, for example, a path change of ca.0.2 mm with a test pressure of 2 bar and a sound pulse duration of 40ms, which signals that the node does not possess the strength requiredfor further processing.

For nodes having a cross section of, for example, 10.5 mm² a path changeof ca. 0.03 mm results if the requisite strength exists. For nodes thatare not fast there results, for example, a path change of ca. 0.1 mmwith a test pressure of 3 bar and a sound pulse duration of 55 ms.

In accordance with the basic representation of FIG. 4, a quality orstrength check can also take place when a compression chamber isdecompressed not only unilaterally, but also multilaterally. It is alonedecisive that a manufactured node 58 is arranged between a counterelectrode 62 that enables an action of pressure and anultrasound-applying sonotrode 60, as can essentially be inferred fromFIG. 4. Moreover, remaining boundary surfaces of the compression chamberthat surrounds the node 58 during welding are formed by lateral sliders64, 66, which are decompressed and laterally moved away during thefurther action of ultrasound and pressure. Corresponding to therepresentation in accordance with FIG. 4, the element 62 can also be acarrier to be welded with the litz wire which in turn is supported onthe electrode or the anvil, which is not represented in connection withthis variant.

The theory according to the invention can moreover be extended such thatshould it turn out in the quality check that a node does not have therequisite strength, a further sound effect takes place with simultaneousaction of pressure, whereby the node 54 almost flows apart and isconsequently destroyed so that consideration in a finishing process isno longer possible.

There also exists the possibility of selectively recompressingsufficiently solid nodes. This can take place when the compressionchamber is open to increase node strength, in other words if furthermoreultrasound impulses are introduced with simultaneous action of pressure.

If the invention has been explained on the basis of the welding of litzwires, this is not intended as a restriction of the theory of theinvention. For example, a verification of the solidity of litz wireswelded to a carrier is also possible. Here the carrier preferably formsa boundary of the compression chamber. Thus the carrier is braced on thecounter electrode. The action of pressure then takes place through thesonotrode. Alternatively the litz wires can be arranged on the sonotrodeand then the carrier can be arranged on this. The action of pressuretakes place via the counter electrode or anvil that is braced on thecarrier.

If a checking of the welded conductors, such as transit or end nodes,has been undertaken on the basis of the previous presented descriptionin the same ultrasound welding device in which the welding of theconductors or the litz wires itself takes place, then there is likewiseno departure from the invention if a quality check is conducted in aseparate machine which has as its main components a sonotrodetransmitting ultrasound vibrations and a counter electrode or an anvilallocated to the sonotrode. Conductors such as transit or end nodes, orlitz wires welded onto a carrier, which are to be checked for theirstrength are then arranged between the sonotrode and the anvil tointroduce an ultrasound pulse with simultaneous action of pressure onthe welded conductors in the previously described manner. At the sametime the change in spacing between the sonotrode and the anvil ismeasured in order to draw conclusions with regard to the strength of thewelding site.

A quality control method of this type has the advantage that conductorswelded in various welding devices can, for example, be checked at acentral location without the normal welding sequence being affected.

1. Method for welding electric conductors (32) such as litz wires usingultrasound, in particular litz wires among one another for producingtransit or end nodes (54, 58) or litz wires with a carrier, whereby theconductors are introduced into a compression chamber (30) that isbounded by at least two boundary elements and are welded after thecompression chamber is closed, whereby ultrasound is applied via a firstelement such as a sonotrode (16) and the conduits to be welded are actedupon by pressure via the first element or a second element, such as acounter electrode (18), and whereby a characteristic magnitude of thecompression chamber is measured, wherein after the conductors (32) arewelded, the compression chamber is decompressed and ultrasound issupplied on the welded conductors, after which the characteristicmagnitude is measured.
 2. Method according to claim 1, wherein thecompression chamber (30) is bounded by at least three elements (16, 18,20), and after the welding, at least one previously fixed and lockedelement (20) in relation to the welded conductor (32) is decompressedand unlatched.
 3. Method according to claim 1, wherein a geometric valuesuch as height, width or diagonals of the compression chamber (30) isselected as a characteristic magnitude, especially the spacing betweenthe first and second elements (16, 18).
 4. Method according to claim 3,wherein the geometric value is measured by a displacement pickup (38).5. Method for quality checking of conductors (32) that have been weldedin a compression chamber (30) of an ultrasound welding device (10),especially into litz wires welded into a transit or end node (54, 56) orlitz wires welded on a carrier, whereby the compression chamber isbounded at least by an ultrasound-applying first element such as asonotrode (16) and a second element such as a counter electrode (18),whereby the conductors are acted upon by pressure via the second orfirst element, characterized by the operations: introducing the weldedconductors (32) to be welded into a compression chamber (30), compactingand welding the conductors (32) with simultaneous cross sectiondiminution of the compression chamber (30), decompressing thecompression chamber (30), whereby the welded conductors (32) remainbetween the first element (16) and the second element (18), renewedapplication of ultrasound with simultaneous action of pressure on thewelded conductors (32) via the first and/or the second element (16, 18)and measurement of a characteristic magnitude of the compression chamberand/or shape of the welded conductors (32).
 6. Method according to claim5, wherein the conductors (32) are subjected to pressure via the secondelement (18).
 7. Method according to claim 5, wherein the quality of thewelding is evaluated as a function of the measured characteristicmagnitude of the compression chamber (30) and/or the shape of the weldedconductors (32).
 8. Method according to claim 5, wherein the heightand/or width and/or diagonals of the compression chamber (30) aremeasured as characteristic magnitudes of the compression chamber (30),for example using a displacement pickup (38).
 9. Method according toclaim 5, wherein the renewed application of ultrasound takes place overa duration T with 10 ms≦T≦250 ms.
 10. Method according to claim 5,wherein the renewed application of ultrasound with simultaneous actionby pressure on the welded conductors (32) takes place with a pressure Pwith 1 bar≦P≦4 bar.
 11. Method according to claim 5, wherein ultrasoundwith simultaneous action of pressure is applied when the compressionchamber (30) is open to destroy or largely destroy the weld when weldsof lesser quality are established on the welded conductors (32). 12.Method according to claim 5, wherein ultrasound with simultaneous actionof pressure is applied again with a decompressed pressure chamber (30)for selective recompression of the weld when proper welding on thewelded conductors (32) is established.
 13. Method according to claim 5for checking the quality of welded conductors, especially welded litzwires such as transit or end nodes or litz wires welded on a carrier,wherein the welded conductors are arranged between a first element thatapplies ultrasound vibrations, such as a sonotrode, and a secondelement, such as a counter electrode, and wherein ultrasound is appliedvia the first element, and changes in spacing between the first andsecond elements with simultaneous action of pressure on the weldedconductors taking place during or after the application are measured.